Large turbine generators are typically cooled with a light density gas. Hydrogen (H2) has been widely used as a coolant due to its desirable thermophysical properties including low windage friction, high heat dissipation capability and high resistance to corona discharge when compared to other cooling gas options. Additionally, H2 has the advantage of being readily accessible and inexpensive.
Among H2 cooled generators there is a type of liquid cooled generators whose inner spaces are cooled with H2 while the stator windings are cooled with a liquid coolant. The stator windings typically include a plurality of hollow copper strands which serve as conduits for the liquid coolant. A typical stator winding includes at least one stator bar coupled to a bar clip and a header coupled to a source of liquid coolant which is conveyed by a pipe through a hydraulic connection to the bar clip. Liquid coolant flows from an inlet coolant header into the flow passages formed by the plethora of hollow copper strands within the stator bar and then flows outwardly into an outlet coolant header for flow into a reservoir. The liquid coolant, such as water, is supplied to the windings via a closed loop system including a heat exchanger and a deionizer.
Liquid cooled generators offer high efficiency, exceptional reliability, quick and simple installation and minimal maintenance costs. However, during operation, the liquid cooled stator windings are subject to an environment of thermal shocks, cyclic duty, corrosion, mechanical vibrations, and electromagnetic stresses which may give rise to the potential for leaks. Leaks in liquid cooled stator windings may originate at any of a number of components, such as copper tubing, pipes, piping connections, among others.
U.S. Pat. No. 7,353,691 teaches a turbine generator leak detection method involving a flow monitoring system that detects H2 leaking into a water cooling system. A variety of methods of leak detection have been used when the leakage is higher than recommended. Among conventional leak test methods, one method involves sniffing detection of H2 in the water tank vent of the SLCS. If a leak is detected, a balloon (bag) test may be conducted to determine the leakage rate. However, the leak locale is difficult to determine during the operation, or during outage service using this approach.
Another method of detecting leaks is to perform pressure decay, and vacuum decay tests to confirm that the stator winding is capable of holding pressure and vacuum. If the pressure within the stator winding falls too rapidly after the stator winding has been pressurized with compressed air, or if the pressure rises too rapidly after air has been evacuated, then a leak that requires further attention is indicated.
If a leak is indicated by pressure and/or vacuum decay testing; then helium tracer gas testing may be used. Helium tracer gas may be performed around all joints such as connection ring to pipes, clip to stator braze joint, water or hydrogen cooled high voltage bushings and a long list of places of interest. Helium tracer gas testing has the disadvantage that it is time-consuming for inspecting the entire winding and requires access to the whole winding. To detect small leaks, the sniffer detector must be brought within 2 to 3 inches of the leak. Because it is nearly impossible to cover every square inch of the winding, tracer test techniques are used to test only the most probable leak sites. Such testing cannot provide confidence that the entire winding is leak-tight.
Conventional leak detection methods are time consuming and, in some cases, may miss some of the leaks. A stator winding that passes a conventional leak test is not guaranteed to be leak free. Additionally, each cycle of the testing requires monitoring for at least 24 hours, if not days, as more than hundreds of those connection and joints may be leak locales. Additionally, conventional methods of leak detection require the detector to be in close proximity to the source of the leak and rely on educated or experienced guesses. This takes considerable time to implement. These methods do not provide a remote, sensitive, accurate, fast detection capability.